1. Field of the Invention
The present invention relates to a device for the transmission of push-pull signals, and more particularly to such a device in which a compensation circuit is provided for the compensation of push-pull signals transmitted by a push-pull transmitter in relation to the inputs of an associated push-pull receiver.
2. Description of the Prior Art
The transmission of dc data signals between remote opposite stations for which identical ground potentials are not assured, is primarily carried out with the aid of push-pull signals across two-wire transmission lines. Differences and mutual fluctuations in the ground potentials then influence the pair of lines as push-pull disturbances, and can be neutralized by a receiver having push-pull suppression facilities. A disadvantage of such an arrangement resides in the fact that two lines, in respect of each transmission direction, are required for every signal. Since it is necessary to transmit a plurality of data signals in parallel, in particular between data processing systems or between the central unit and the periphery of the system, a considerable expense is involved in respect of lines and connecting plugs.
The line requirement can be reduced with the aid of devices which facilitate data transmission in both directions across a two-wire transmission line, i.e. full duplex operation. In this case it is necessary to prevent the output signal of a transmitter from reaching the assoicated receiver. It has long been known to interpose the transmitter and receiver of every opposite station into the diagonals of a bridge circuit, and to arrange the two-wire transmission line and a balancing line into two adjacent bridge arms. However, this type of bridge circuit has the disadvantage that only a small part of the transmitter power is applied to the transmission line, and likewise only a small portion of the power of the incoming signal is available at the receiver input.
The German published application No. 2,045,654 discloses a circuit arrangement for the full duplex transmission of dc data signals by way of a two-wire transmission line, wherein the transmitter, a compensation circuit, and the receiver are connected in series to the two-wire transmission line. The output signal of the compensation circuit compensates the signal from the associated transmitter which would otherwise be applied to the receiver. The compensation circuit has a comparatively complicated construction in order to be able to balance deformations of the transmitted signal, as a result of the generally complex input impedances of long, and therefore dissipative transmission lines. However, the compensation circuit can be considerably simplified if only lines whose input impedances are real, with good approximation, are used.
The series arrangement of transmitter, receiver and compensation circuit in the known transmission device gives rise to asymmetry in respect of ground potentials, which is very harmful. This applies in particular to the two-wire transmission line. Fed-in interference voltages now become manifest as push-pull interferences, which can no longer be eliminated by a receiver which has good push-push suppression facilities.